Interstitial, subhedral, slightly pink zircons have been separated from a diorite of the External Gabbro unit of the Finero Mafic Complex, which crops out in the northernmost tip of the Ivrea^Verbano Zone (IVZ, Southern Alps).They are characterized by oscillatory zoning under cathodoluminescence (CL), a highTh/U ratio (0·9^0·4), and associated high U, Th, Pb, rare earth element (REE),Ta, Nb, Li and P contents, consistent with a magmatic origin.The mean of the single concordant ages is 2323Ma, which is here proposed to constrain the timing of intrusion of the Finero Mafic Complex. Most of these magmatic zircons exhibit an outer rim, up to 50mm thick, with a bright emission, with transgressive and/or sub-concordant contacts to the internal, oscillatory-zoned domains, in which the Th/Uratio is decreased, as are the U,Th, Pb, REE,Ta and Nb contents. These rims give younger, slightly discordant 206Pb/ 238U ages, between 2193 Ma and 2053 Ma, which are interpreted to highlight a fluid-assisted recrystallization event. The gabbros also have an interstitial zircon population consisting of rounded, small, colourless, bright to milky crystals with blurred zoning patterns in CL. These zircons have low U, Th, Pb, Nb and Ta contents, and lowTh/U ratio (down to 0·08).The overall metamorphic character, together with field evidence of the presence of blocks of paragneiss belonging to the crystalline basement at the roof of the External Gabbro unit, indicates that these zircons are inherited from local crustal rocks. They have U and Pb isotopic compositions that are strongly altered and discordant, showing an upper limit of the timing of recrystallization consistent with the MiddleTriassic emplacement age obtained from the magmatic zircons. It is concluded that the Finero Mafic Complex can no longer be considered as a part of the huge Permian Mafic Complex of the central IVZ. It follows that the intrusive record preserved in the IVZ allows us to look at the interplay between the continental crust and mantle melts through time (i.e. Permian toTriassic). In this scenario, the modest crustal assimilation shown by the Finero Mafic Complex can be interpreted in the first instance as the consequence of the limited volume of intruded melt (less than 2 km in thickness). However, the possible role played by the decreased fertility of the metamorphic country rocks owing to a inferred Permian magmatic event must be better constrained.The intrusion of the Finero Mafic Complex may represent the deep-crustal counterparts of the Ladinian volcanism that is widespread throughout the Southern Alps. The geodynamicsetting of this magmatism is still strongly debated.The results presented in this study, along with several other structural, petrochemical and age heterogeneities documented in the literature, suggest the occurrence of a ‘Finero-type IVZ’ (i.e. northern IVZ) and a ‘Balmuccia-type IVZ’ (i.e. central IVZ), with different magmatic and tectonic evolutionary histories.

SHRIMP U^Pb ZirconTriassic Intrusion Age of the Finero Mafic Complex (Ivrea^Verbano Zone, Western Alps) and its Geodynamic Implications

MAZZUCCHELLI, MAURIZIO;SINIGOI, SILVANO;
2013

Abstract

Interstitial, subhedral, slightly pink zircons have been separated from a diorite of the External Gabbro unit of the Finero Mafic Complex, which crops out in the northernmost tip of the Ivrea^Verbano Zone (IVZ, Southern Alps).They are characterized by oscillatory zoning under cathodoluminescence (CL), a highTh/U ratio (0·9^0·4), and associated high U, Th, Pb, rare earth element (REE),Ta, Nb, Li and P contents, consistent with a magmatic origin.The mean of the single concordant ages is 2323Ma, which is here proposed to constrain the timing of intrusion of the Finero Mafic Complex. Most of these magmatic zircons exhibit an outer rim, up to 50mm thick, with a bright emission, with transgressive and/or sub-concordant contacts to the internal, oscillatory-zoned domains, in which the Th/Uratio is decreased, as are the U,Th, Pb, REE,Ta and Nb contents. These rims give younger, slightly discordant 206Pb/ 238U ages, between 2193 Ma and 2053 Ma, which are interpreted to highlight a fluid-assisted recrystallization event. The gabbros also have an interstitial zircon population consisting of rounded, small, colourless, bright to milky crystals with blurred zoning patterns in CL. These zircons have low U, Th, Pb, Nb and Ta contents, and lowTh/U ratio (down to 0·08).The overall metamorphic character, together with field evidence of the presence of blocks of paragneiss belonging to the crystalline basement at the roof of the External Gabbro unit, indicates that these zircons are inherited from local crustal rocks. They have U and Pb isotopic compositions that are strongly altered and discordant, showing an upper limit of the timing of recrystallization consistent with the MiddleTriassic emplacement age obtained from the magmatic zircons. It is concluded that the Finero Mafic Complex can no longer be considered as a part of the huge Permian Mafic Complex of the central IVZ. It follows that the intrusive record preserved in the IVZ allows us to look at the interplay between the continental crust and mantle melts through time (i.e. Permian toTriassic). In this scenario, the modest crustal assimilation shown by the Finero Mafic Complex can be interpreted in the first instance as the consequence of the limited volume of intruded melt (less than 2 km in thickness). However, the possible role played by the decreased fertility of the metamorphic country rocks owing to a inferred Permian magmatic event must be better constrained.The intrusion of the Finero Mafic Complex may represent the deep-crustal counterparts of the Ladinian volcanism that is widespread throughout the Southern Alps. The geodynamicsetting of this magmatism is still strongly debated.The results presented in this study, along with several other structural, petrochemical and age heterogeneities documented in the literature, suggest the occurrence of a ‘Finero-type IVZ’ (i.e. northern IVZ) and a ‘Balmuccia-type IVZ’ (i.e. central IVZ), with different magmatic and tectonic evolutionary histories.
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